Update libsysdecode for getfsstat() 'flags' argument changing to 'mode'.

As a followup to r310638, update libsysdecode (and kdump) to decode the
'mode' argument to getfsstat(). sysdecode_getfsstat_flags() has been
renamed to sysdecode_getfsstat_mode() and now treats the argument as an
enumerated value rather than a mask of flags.